The researches on the friction properties of nc-WC/a-C : H coating on the lightweight valve stem

The lightweight valves, especially made of TiAl alloys, are more and more often used in modern combustion engines with cam and camless valvetrain. To increase the wear resistance and to decrease the coefficient of friction between valve steam and valve guide, different kinds of the coating are used. The researched valve has been outlet one made of TiAl6Zr4Sn2Mo2 alloy with the original chrome coating on the valve stem. The nc-WC/a-C:H nanocoating has been deposited on the chromed stem of researched valve, which has mated with the guide made of cast iron. The aim of the present paper is to investigate friction properties of such nc-WC/a-C:H deposited onto the chromed and polished surface of the valve stem. The scheme of research stand has been presented in the paper. On the stand it has been possible to measure values of valve lift and of valve acceleration, the impact force between valve and its seat insert, the friction force between valve stem and its guide, the temperature values for valve guide and for seat insert and a sound level during impacting. Researches have been performed in conditions of room temperature and without oil lubrication in the valve stem - guide contact zone. Basing on the measured values of friction force between valve stem and its guide vs. time and loading frequency, the coefficient of friction has been estimated. Obtained curves of friction force and of friction coefficient vs. time, for different frequency values of valve loading have been presented in the article.